This invention relates generally to a method and apparatus for channel estimation. More particularly, this invention relates to a method and apparatus for transforming a channel estimate obtained for one modulation type to the corresponding channel estimate for another modulation type.
In a communication system such as a cellular communication system including a transmitter, e.g., a base station, and a receiver, e.g., a mobile station or a fixed cellular terminal, information exchanged between the transmitter and receiver contains data bits and synchronization bits. The information is modulated for transmission.
In some cellular communication systems it can be an advantage to use one modulation type during a first portion of the transmitted signal, e.g., the portion used for synchronization and channel estimation, and another modulation type for a second portion of the transmitted signal, e.g., the portion containing the data to be transmitted. This allows the modulation types to be chosen for efficient transmitter implementation. For an efficient transmitter architecture, an offset is typically introduced in the modulation format. For instance for an 8-PSK signal, the transmitted signal can be given by:                               u          t                =                              ⅇ                          j              ⁢              π              ⁢                              xe2x80x83                            ⁢                              t                8                                              ⁢                      v            t                                              (        1        )            
where                                           v            t                    =                      ⅇ                          j              ⁢                              π                4                            ⁢              k                                      ,                  k          ∈                      (                          0              ,              1              ,              …              ⁢                              xe2x80x83                            ,              7                        )                                              (        2        )            
The offset here means that the symbol constellation rotates xcfx80/8 radian between adjacent symbols. This rotation produces a transmitted signal with low peak-to-average ratio and no zero-crossing, which allows for an efficient transmitter architecture.
On the receiver side, the received signal has to be de-rotated in order to correctly detect the received symbols. However, due to multi-path propagation of the radio waves, the rotation of the received signals results in a radio channel estimate that is a function of the offset rotation.
Information in cellular radio systems is typically transmitted in bursts containing a synchronization signal, such as a training sequence, and an information sequence. If the whole burst is transmitted with the same modulation format, there are no negative effects caused by the channel estimate being a function of the offset rotation. However, if one modulation type, with a specific offset, is used for the training sequence and another modulation type, with another offset, is used for the information sequence, the channel estimate obtained for the first modulation type must be transformed to a correct channel estimate for the second modulation type.
There is thus a need for an efficient technique algorithm that transforms a channel estimate obtained for one modulation type to the corresponding channel estimate for another modulation type.
It is therefore an object of the present invention to provide an accurate channel estimate for a received signal when different parts of the received signal are modulated by more than one modulation type.
According to exemplary embodiments, this and other objects are met by a method and apparatus for estimating a channel in a received signal, wherein different parts of the received signal are modulated by different modulation types. The channel is estimated based on a part of the received signal modulated using a first modulation type. The estimated channel is then transformed into a channel estimate corresponding to at least a second modulation type.